Many coating formulations, such as paints, varnishes, lacquers and inks, contain volatile organic compounds which are evaporated to dry or cure the coating. The general practice has been to allow the volatile compound to pass into the atmosphere. In many industrial coating applications, heated air is used to speed up the curing or drying time.
There are new incentives to change present practices. The emissions of volatile organic compounds (VOC) are being regulated to improve the ambient air quality in the United States. The recent price increases for petroleum and its derivatives has greatly increased both the cost of VOC's and the cost of energy to heat the air to speed the drying or curing.
In setting up the guidelines for regulation of VOC emissions, the U.S. Environmental Protection Agency (EPA) has research into "Reasonalbly Available Control Technology" (RACT). (See EPA-450/2-77-008-OAQPS, No. 1.2-073.) Three approaches were judged to be available. These are:
1. Reformulation, either to substitute water for VOC or to use high-solid, low-solvent formulations. PA1 2. Recovery, where activated carbon is used to adsorb VOC and steam is used to expel the VOC from the carbon so the carbon can be reused. PA1 3. Incineration, where the VOC vapors are burned at high temperatures to carbon dioxide.
Those who are "familiar with the art" know that tradeoffs are involved, or there are penalties associated with each of the above approaches. The reformulated coatings may be more difficult to apply, or have poorer use characteristics, or be slower drying when formulated with water instead of a more volatile organic compound. Recovery using carbon is relatively expensive; the carbon may be fouled with high boiling compounds, ineffective for alcohols, and subject to spontaneous ignition. The cost to recover a wet mixture of solvents in many cases will exceed the cost of purchasing new solvents. Incineration is also relatively expensive, but more able to accommodate any mixture of solvents (or VOC) without disrupting present quality. Additional fuel (or energy) is generally needed to heat the VOC air mixture to a combustion temperature.
This invention is to make the recovery of VOC more economical and to conserve energy in those operations where a drying oven or a curing oven is used. It is applicable where sheet metal, paper, film, or textile materials (any web) is coated or printed and passed into an oven to speed up the removal of any volatile organic compounds. It is also applicable where webs or various parts, shapes, or objects are moved through an oven to facilitate either a drying or curing reaction with the incidental release of organic vapors. For instance, formulated compositions such as brake shoes and clutch plates may be dried to remove alcohol or simultaneously cured with the progression of a chemical reaction. Some curing operations, such as setting of a foamed plastic coating can lead to the incidental vapor emission of a relatively high-boiling plasticizer.
It is well known that explosion prevention is very important in the operation of any drying or curing oven. Most commonly, excess air is blown through an oven to prevent the organic vapor level from approaching too closely to the lower explosive limit (LEL).
This invention is directed to VOC recovery by vapor condensation, wherein gases from the relatively warm oven are passed through a condenser and returned to the oven. This approach is not new, but it has not been commonly practiced or considered. U.S. Pat. No. 1,075,586, issued in 1913, describes the recirculation of a dryer gas through condenser and back to the dryer. In 1923, Smith was granted U.S. Pat. No. 1,470,650 for a similar arrangement where the recirculation rate was controlled to prevent the build-up of explosive compositions. Subsequently, it has become common knowledge that an inert gas such as nitrogen, instead of air, can be supplied to maintain non-explosive compositions in a dryer or oven. Relatively recently, in 1979, Rothchild was granted U.S. Pat. No. 4,150,494 for a system wherein a mixture of vapor and inert gas contained in the oven is withdrawn through a relatively cold condenser (to condense at least 85% of the vapor) and a major portion of the non-condensed gas and vapor is discharged to the atmosphere. The inert gas supplied to this system is shown to flood the outside of both the entrance and exit ports of the oven so that inert gas rather than air is drawn into the oven. Cryogenic or refrigerated condenser temperatures, such as those achieved with liquid nitrogen are indicated to be preferred.
It is obvious that the efficiency of a vapor condensation process is increased if the concentration of vapor in the oven atmosphere is increased, but on the other hand, the drying rate may be decreased or stopped if the vapor concentration is too high. In most cases, a small increase in oven temperature can be made to increase the drying rate. In all cases, drying will occur when the oven temperature is above the boiling point of the VOC. Trials to demonstrate Rothchild's process have proven that satisfactory drying rates can be maintained even with relatively high VOC vapor concentrations in the oven. In those cases where a residual solvent odor is unacceptable, a second stage or final drying with excess air may be needed, but the relatively small amount of VOC removed at this stage generally is not worth the cost that would be required for its recovery.
If a drying oven could be hermetically sealed, all VOC liquid in the warm oven chamber could be vaporized and transported to a relatively cool condenser. However, all ovens designed for continuous operation necessarily have an entrance port and an exit port. When the temperture inside an oven increases and/or when the average composition changes to include more VOC vapor relative to non-condensable gases, the normal effect is an increased internal pressure with a resultant flow of gas and VOC vapor out through the port openings. Conversely, when the temperature in an oven decreases and/or the average composition changes to less vapor, the normal effect is for a decreased internal pressure and a flow of external atmosphere through the port openings.
Even with a well operated continuous oven, it is not practical to always maintain constant temperatures and vapor compositions in the closed system; therefore, there is a tendency for alternate inhalation of external atmosphere and exhalation of oven atmosphere which includes VOC vapor.